Most conventional techniques for detecting ionizing radiation (gamma rays, x-rays, and other energetic particles) with solid-state semiconductor devices rely on two-terminal devices. Reference in this regard may be had to commonly assigned U.S. Pat. No. 5,391,882, entitled "Semiconductor Gamma Ray Detector Including Compositionally Graded, Leakage Current Blocking Potential Barrier Layers and Methods of Fabricating the Detector", by David R. Rhiger, a co-inventor of the subject matter of this patent application.
In a solid-state detector the ionizing radiation produces electron-hole pairs within the semiconductor material, which then move under the influence of an electric field toward their respective contact terminals (electrons towards a positive terminal and holes towards a negative terminal). However, because holes have a mobility that is approximately 10 times less than that of electrons, they are more easily trapped before reaching the negative contact. This results in an undesirable condition known as incomplete charge collection.
In greater detail, and in all solid-state radiation detectors that are known to the inventors, the absorbed radiation produces electron-hole pairs, or charge carriers. For example, a 100 keV gamma ray, when absorbed in the Group II-VI alloy semiconductor material CdTe, produces about 22,000 electron-hole pairs through a cascade effect from a primary photo-electron. The number of generated electron-hole pairs is directly proportional to the energy of the gamma ray. The charge carriers then drift in an electric field toward their respective contacts. A signal in an external circuit, that is connected to the contacts, arises as a result of the fact that energy is given up by the applied electric field for moving the charge carriers. For example, when falling through a potential difference of 200 volts, an electron extracts 200 eV of energy from the electric field. To replace this energy in the electric field and to maintain a constant applied voltage potential, a voltage supply in the external circuit produces a current to the detector contacts. It is this current, which flows in response to the motions of charge carriers within the detector, that constitutes the signal in the external circuit. When every carrier that is generated by the gamma ray is able to move or drift the full distance from its point of generation to its respective contact, then a full output signal is produced. This is the most desirable condition of 100% charge collection efficiency. In this case, every incoming gamma ray of the same energy will result in an output signal (pulse) of the same amplitude (except for a small spread due to the statistics of the charge generation mechanism). When a number of pulses is plotted versus the pulse amplitude, a very narrow peak is displayed. When performing high resolution spectroscopy the 100% charge collection state is an important precondition for obtaining accurate and repeatable results.
A problem to be solved in order to approach the 100% charge collection state results from the fact that not all of the charge carriers can reach their respective contacts because of trapping within the semiconductor material. The trapping times, t.sub.e and t.sub.h for electrons and holes, respectively, are defined as the average time that a charge carrier survives before falling into a trap. On an energy diagram such traps are located in a gap between the conduction band and the valence band of the semiconductor material. Once trapped, the charge carrier may be subsequently released from the trap, or it may be recombined with a charge carrier of the opposite polarity. In either case the contribution that the charge carrier would have made to the signal in the external circuit is either significantly delayed or eliminated altogether.
Quantitatively, the differences between the two kinds of charge carriers can be described as follows. In a given semiconductor material the electrons and holes have the respective mobilities .mu..sub.e and .mu..sub.h. Under the influence of an electric field, of strength E, their drift velocities become EQU v.sub.e =.mu..sub.e E and v.sub.h =.mu..sub.h E,
respectively. Due to trapping times, the drift length (L) (average distance each carrier can travel) is given, respectively, by EQU L.sub.e =v.sub.e t.sub.e =.mu..sub.e t.sub.e E, and EQU L.sub.h =v.sub.h t.sub.h =.mu..sub.h t.sub.h E.
When comparing the two drift lengths it should be noted that the mobility of electrons is approximately 10 times greater than that of holes, while the trapping times of electrons are two to five times greater than for holes. Thus L.sub.e is about 20 to 50 times greater than L.sub.h. By example, and for the Group II-VI compound semiconductor material CdZnTe, a typical value for the .mu..sub.e t.sub.e product is 1.times.10.sup.-3 cm.sup.2 /V.
With a typical electric field strength of 1000 V/cm, it can readily be shown that L.sub.e =1 cm. Because this drift length is long compared to a typical CdZnTe gamma ray detector thickness of 0.1 cm or 0.2 cm, the electrons can be collected with very high efficiency. The holes, however, due to their significantly smaller drift length, travel a much smaller distance than the detector thickness before being trapped. The resulting incomplete collection of charge carriers in the detector leads to unpredictable errors in the output signal, and hence a broadening of the pulse height spectrum away from the ideal sharp peak.
The inventors are aware of three conventional approaches for attempting to overcome the problem of incomplete charge collection. A first approach increases the electric field strength so as to increase the average charge carrier velocity. However, increasing the electric field strength also increases the leakage current of the solid-state detector. A second approach absorbs the ionizing radiation near the negative contact, thereby reducing the distance that the holes must travel in order to be collected. It has been found, however, that this approach is not suitable when detecting higher energy radiation (e.g., gamma rays over about 20 keV). The third approach employs a non-planar shape for the semiconductor material in order to produce a larger magnitude electric field strength only in the vicinity of the negative contact. However, this latter approach involves difficult and expensive device fabrication techniques.
A conventional Frisch grid ion chamber three-terminal detector (not a solid-state detector) is illustrated schematically in FIG. 1. Reference in this regard may also be had to "Radiation Detection and Measurement", Second Edition, G. F. Knoll, John Wiley & Sons (1989), pages 149-157. A gridded ion chamber or tube 1 contains a low pressure gas between a cathode 2 and an anode 3. An intermediate grid, known as the Frisch grid 4, divides the tube 1 into two regions. Due to the use of shielding (not shown) an incident gamma ray impinges only on the region between the cathode 2 and the Frisch grid 4, where it generates free electrons and positive ions from the gas molecules. The positive ions move toward the negative potential that is applied to the cathode 2 with batteries 5a and 5b. Because the ions are collected more slowly and experience greater recombination, they would be detrimental to the signal in regards to the total output and timing. However, the electrons are attracted towards the Frisch grid 4 and pass through it, and then continue to drift towards the anode 3 which is positive with respect to the cathode. An external signal (pulse out) is obtained across R.sub.L between the anode 3 and the Frisch grid 4 and, as a result, only the electrons contribute to the output signal from this circuit. That is, a slow rise in the output pulse corresponding to the drift of the ions is eliminated, and the signal rise time corresponds to that of the significantly faster electrons. Since each electron passes through the same potential difference between the Frisch grid 4 and the anode 3, and contributes equally to the output pulse, the pulse amplitude is made independent of the position of formation of the original electron-ion pairs. In this case the pulse amplitude becomes proportional to the total number of ion pairs formed along the track of the incident particle or ionizing radiation.
As shown by the plot of signal versus time in FIG. 2, the signal appears only when the group of electrons is drifting between the grid and anode. In FIG. 2 n.sub.o e is the number of electrons, y is a distance from the Frisch grid 4 where the incident radiation is absorbed, d is the spacing between the Frisch grid 4 and the anode 3, C is the capacitance from the Frisch grid 4 to circuit ground, and v.sup.- is the velocity of the electrons.
Reference may also be had to an article entitled "Single-polarity charge sensing in ionization detectors using coplanar electrodes", P. N. Luke, Apply. Phys. Lett. 65(22), pgs. 2884-2886, 28 Nov. 1994, which describes a multi-terminal solid-state detector that makes use of co-planar electrodes to separate charge carriers produced by the ionizing radiation. In this device interdigitated coplanar grid electrodes are applied to a surface of a CdZnTe detector, and a common electrode is applied to an opposite surface. Perceived disadvantages to this approach include a non-uniformity of the electric field, and a difficulty in applying this device geometry to the fabrication of an array (e.g., a two-dimensional array) of ionizing radiation detectors having a small pixel size. Moreover, leakage current in the entire coplanar circuit contributes to the output signal.